Field of the Invention
The invention relates to a high precision flow meter for measuring a gaseous volume flow in a pipe. In particular, the invention relates to a high time resolution flow meter for measuring the volume flow of an exhaust gas in a pipe of an internal combustion engine.
In the prior art, flow meters are used for monitoring the time history of an exhaust gas flow of an internal combustion engine during various testing cycles of the engine, in which different driving conditions of a motor vehicle are simulated.
From the dissertation of Mr. Andreas Hess, Langstra.beta.e 18, 7526 Weiher, Germany, titled "Sensor for Dynamic Volume Flow Determination in a Diluted Exhaust Gas of a Motor Vehicle", which was written in co-operation with the Politechnic Mannheim, Department Communication Engineering, dated 1993, it is known to determine the volume flow of an exhaust gas by measuring the run time differences of ultrasonic signals travelling obliqely through the volume flow from a first sonic transmitter to a first sonic receiver and substantially in the opposite direction from a second sonic transmitter to a second sonic receiver on the basis of the so-called "run time difference method". According to this method, the average velocity of the volume flow of the exhaust gas in a pipe is determined on basis of the angle between the travelling path of the sonic signals and the volume flow and the length of the travelling path by measuring the run time differences of the sonic signals which are caused by an addition or a substraction of the velocity of the volume flow and the associated components of the speed of the sonic signals, respectively. By multiplying the determined velocity of the volume flow and the cross section of the pipe, the instantaneous average volume flow of the exhaust gas can be determined. In the apparatus described in the dissertation, piezo crystals which are disposed opposite to each other, are used for generating and receiving the sonic signals. Each of the piezo crystals is operated as a transmitter and a receiver at substantially the same time. Due to transient effects or phenomena which are caused as a result of the inertia of the piezo crystals, and which leads to a comparatively large error in the determination of the exact measuring time at which a sonic signal is transmitted or received, the described apparatus has a comparatively low accuracy in determining the volume flow. Moreover, due to the unfavourable vibrational properties of the piezo crystals, the apparatus does not allow for measuring a volume flow with a sufficient time resolution, as it is required for determining and verifying the amount of pollutants generated during a testing cycle. Moreover, it is a disadvantage of the described apparatus that the piezo crystals are in direct contact with the corrosive and hot exhaust gas. As a result of the direct contact between the crystals and the exaust gas, the operational life time of the crystals is strongly reduced and a further measurement error is caused by the thermal behaviour of the piezo crystals.
In addition, the "run time difference method" is also described in the article titled "Dubbel", Edition 18, page W 16, Springerverlag.